Before you can be certified to teach any subject or grade level, you are required to complete at least 10 weeks* of apprentice teaching with a master teacher.
The process begins this way:
Your college or university contacts the school districts in your area and says, "want some free labor in exchange for wisdom?" The school district says, "that sounds pretty good... let me see if I have any takers." Eventually they do and we're on our way.
The school district finds an interested teacher, usually someone who's been teaching at least 5 years, and introduces you. This person may be dedicated to teaching and love sharing his/her gifts, or it can be a person who's completely fed up and wants to take a load off. It's the luck of the draw.
Your college or university sets out general guidelines for hoops you must jump through and the manner through which these hoops must be jumped. Then they choose a university supervisor for you, a professor who can pop in from time to time to evaluate your progress and answer any questions for you. Ready, set, go.
What some schools put more plainly than others is this: you are now at the mercy of your cooperating teacher, some of whom are more cooperative than others. Tread lightly.
Over the next 10 weeks, or semester, or year, depending on your university, you will work as a teacher. You will remain at school for the duration of the contact hours, typically at least half an hour before and after school. You will take on the duties of your master teacher, including detention, hall monitoring, bus and cafeteria duty. You will be expected to grade papers, teach lessons, and manage the classroom.
At the end of your stay, you will shed the word "student" or "apprentice" and become what you've been working at for so long: a teacher.
*Some states require much more than 10 weeks. Almost no one requires less. P.S. sorry for being US-centric, but node what you know.